1. Field of the Invention
The present invention relates to a group III nitride semiconductor device (also referred to herein as a device) and, particularly to a process for producing the same.
2. Description of the Related Art
In the art of a luminous device such as a light emitting diode, a semiconductor laser diode or the like, there is known a luminous device or opto-electronic device comprising a crystal layer including a single crystal of group III nitride semiconductor (Al.sub.x Ga.sub.1-x).sub.1-Y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) to which group II elements such as magnesium (Mg), zinc (Zn) or the like are added, which is attractive as a wide gap semiconductor expected to be a material for a device capable of emitting blue light.
A group III nitride crystal made of aluminum (Al), gallium (Ga), indium (In) and nitrogen (N) [(Al.sub.x Ga.sub.1-x).sub.1-Y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1)] to which group II elements such as Mg, Zn or the like are added, is produced by the metal-organic chemical vapor deposition (MOCVD) which is normally used for epitaxial growth of the nitride semiconductor. This so-called group II added group III nitride crystal as it is immediately after epitaxial growth has a high resistance. Therefore, even if a blue light emitting diode is produced in the basis of group II added group III nitride crystal layers, it is difficult to provide an electric current to the as grown group II added group III nitride.
Recently, a reforming method have been reported that a high resistance (Al.sub.x Ga.sub.1-x).sub.1-Y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) crystal to which Mg or Zn is added is reformed to a low-resistivity p-type crystal by means of a specific treatment. H. Amano et al. discloses that a low-resistivity p-type crystal is established by performing a low energy electron beam irradiation treatment to such a crystal (H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. phys. Vol. 28, 1989, pp-L2112-L2114). Further, S. Nakamura et al. also discloses that a low-resistivity p-type crystal is achieved by performing a thermal annealing treatment under a pressurized or atmospheric pressure in an atmosphere of nitrogen to such a crystal (S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. Vol. 31, 1992, pp-L139-L142).
The mechanism of the foregoing treatments for establishing the p-type layer is interpreted as follows: The hydrogen atoms are combined to the group II acceptor impurities such as Mg or the like in the layer formation and neutralizing the acceptors. There ocuur dissociation and elimination of hydrogen atoms due to the above treatments.
The foregoing low energy electron beam irradiation treatment provides an excellent p-type GaN film having a high hole carrier concentration at room temperature of E18/cc order. However, the treated depth is restricted by the penetration depth of the electron beam. In the case that the accelerating voltage for electrons of from 6 kV to 30 kV is used, the treated depth is approximately 0.3 .mu.m from the surface of GaN film as described in Nakamuraet. al. (S. Nakamura, T. Mukai, M. Senou and N. Iwasa, Jpn. J. Appl. Phys. Vol. 31, 1992, pp-L139-L142). In addition, the electron beam is scanned on the surface of the wafer one by one in the low energy electron beam irradiation treatment, so that the increase of the treatment time per one wafer causes problems in the mass production of the device.
On the other hand, the foregoing thermal annealing treatment does not restrict any treated depth caused by the penetration depth of the electron beam. Furthermore, the thermal annealing treatment is advantageous in the mass production of the device, since a plurality of the wafers can be introduced into a heating furnace to be performed the annealing process. There is however a drawback in the thermal annealing treatment discovered by Nakamura et. in which, as seen from the Nakamura's experiment, the resultant hole carrier concentration at room temperature of 3E17/cc is lower than that of the low energy electron beam irradiation treatment. The value of 3E17/cc hole carrier concentration at room temperature is enough to produce a p-n junction diode which is a basic element of the light emitting diode or semiconductor laser and thus such a thermal treatment is used in practical for the manufacturing of emitting diodes.
In case of the manufacturing for the semiconductor laser device using the particular thermal treatment, a problem of contact resistance at the electrode portion remains due to the hole carrier concentration at room temperature of about 3E17/cc. Moreover, when raising the treatment temperature in expectation of increasing the hole carrier concentrations at room temperature of the whole films, and then nitrogen atoms are dissociated adjacent to the outer surface of the film to generate vacant holes of nitrogen, so that vacant holes of nitrogen serve as donors to compensate acceptors. On the contrary to the expectation, the hole carrier concentration is reduced adjacent to the outer surface of the film. As a result, the contact condition at the electrode portion becomes inferior. In addition, the elevation of the annealing temperature readily promotes a mutual diffusion between the internal matrix elements of the device and the acceptor impurities of the growth layer.